Environmental control systems for aircraft typically employ air cycle machines and heat exchangers to cool and condition high pressure air supplied by either the engines or an auxiliary power unit. In these systems, compressed supply air is further compressed in a compressor and cooled in a heat exchanger, and then expanded in a turbine. The turbine outlet air, cooled by expansion, then flows into the aircraft. Since the aircraft air is maintained at a lower pressure than the supply air, properly designed systems provide conditioned air at temperatures low enough to cool both the cabin and the aircraft avionics.
In the typical air cycle machine, a pair of hydrodynamic journal bearings are used to regularly locate and support the shaft. For optimum machine performance, very small clearances between the stators, fixed to the machine housing, and the tips of the compressor and turbine blade must be maintained. Since the compressor and turbine rotors, to which the blades are attached, are connected to the shaft, if the bearings allow more than slight amounts of radial free play then the shaft would shift when loaded and the blade tips would contact the stator surfaces encircling them, causing damage to the machine.
In a first type of air cycle machine, each journal bearing is mounted in a separate segment of the turbine and compressor housings. These housings are generally made from aluminum and have thick wall portions. In machines of this type, a journal bearing is mounted directly in each separate segment of the housing. Bearing alignment is achieved by machine matched bores in the housing. However, as the machines use dissimilar metals in the housing (aluminum) and the rotating members (stainless steel), the clearance between rotating members and the housing are very critical. Failure of this type of machine can be very expensive because both the rotating member and the entire housing must be replaced if the bearings fail. See, for example, U.S. Pat. Nos. 4,725,206, 4,786,238, and 4,503,683.
In another type of machine a journal bearing shell is disposed between the bearings mounted on the rotating member and those mounted on the stationery housing. As they offer minimal freeplay and reliable operation at high speed, hydrodynamic fluid film journal bearings are used to locate the rotating shaft member radially. The journal bearing shell provides better thermal compliance and improved mechanical dampening, and is more cost effective if a bearing failure occurs since only the journal shells and bearing, and not the entire turbine and compressor housings must be replaced. In machines of this type, however, since each journal bearing is mounted in a separate segment of the housing, matched sets of journal shells and machine portions of the rotating shaft must be provided. An inner race of each bearing connects to, or is part of a shaft, while the outer race of each bearing attaches to the housing through the shell. The correct clearance must be maintained between the inner and outer races to ensure that the magnitude of the hydrodynamic bearing forces remain constant during operation. Thus, the most important operating parameter for hydrodynamic journal bearings is journal alignment. Any misalignment of the journal bearing with respect to the axis of rotation of the rotative assembly will cause radial loads to be generated and these radial loads limit the life of the machine, or if they become excessive can cause immediate failure of the machine. An example of this type of machine comprising two journal bearings in two different bearing shells can be found, for example, in U.S. Pat. No. 5,113,670. Since two journal bearings are used in this type of machine to support the shafts, both bearings' center line must be located as close to the same axis as possible. In these machines, each journal bearing is mounted in a separate segment of the housing. Pilots, matched sets of two corresponding machine annular surfaces located at the outer most diameter where each housing segment contacts another segment, maintain the relative radial locations of adjacent segments. In each matched pilot set, a first surface is located on the inside of the outer housing wall of one segment. A second surface, with included O-ring glands, is located on the outside of the outer housing wall of the adjacent segment. When assembled, the second surface sets inside the first, and the machine first inner surfaces rests on an O-ring installed into this gland, fixing the relative radial orientation of these two segments. If the pilots on each housing segment are concentric with the journal bearing bore, when the housing is assembled, perfect machine alignment is achieved. However, given the large diameter of the pilots relative to the bearing diameters, machining pilot surfaces onto the housing segments to such close tolerances is both difficult and expensive. In an effort to overcome this problem an alignment fixture has been disclosed in U.S. Pat. No. 5,142,762. While this approach ensures improved bearing alignment, it also requires that after the fixture is used to obtain the relative orientation of each segment, the segments be disassembled to remove the fixture and then the segments reassembled to form a completed air cycle machine.
In turbo charger machines, which include a compressor wheel and a turbine wheel mounted on opposite ends of a shaft supported by force lubricated ball bearings, the bearings have been provided with a cartridge bearing assembly which is located within the turbo charger housing. Since in this type of rotating machine it is necessary to permit shaft excursions when the rotational speed of the shaft passes through critical frequencies, the use of lubricating fluid, such as engine lubricating oil, is employed to both lubricate the ball bearings and also to provide a film of fluid between the cartridge and the housing. To accurately locate the bearing cartridge within the turbocharger housing, it has been disclosed to use a pin which is received in coaxial apertures in the housing and in the outer ring of the bearing cartridge to more accurately locate the bearing cartridge and therefore the shaft, within the turbocharger housing. See, for example, U.S. Pat. No. 5,076,766. Protecting such ball bearings from excessive heat is normally achieved by providing the ball bearings with adequate supply of lubricating oil from a pump driven by an engine, when the engine is running. However, this supply of cooling oil ceases when the engine stops and such ball bearings are vulnerable to the effects of heat-soak which can damage the ball bearings adjacent the turbine rotor.
Gas lubricated bearings have been considered to overcome the problem of lubricant breakdown, especially that which is due to soak-back after shutdown of a turbo compressor with ball bearing cartridges. In general, turbo compressor machines utilizing gas lubricated bearings use dissimilar metals in the rotating portions and the stationary housing portions. The rotating portions are generally constructed from material with higher thermal conductivity, such as low alloyed steels, while the housing portions are constructed from lightweight material, such as aluminum. The use of the dissimilar metals causes differences in the thermal expansion of the metals which has an effect on the tolerances of the bearings used. In order to compensate for this differential thermal expansion, prior art machines locate the bearing assemblies directly within the housing. See, for examples, U.S. Pat. Nos., 4,503,683, 4,725,206, and 4,786,238. This type turbo compressor machine eliminates the use of journal shells, but creates the problems of poor thermal compliance poor mechanical dampening and more costly repairs on failure of the machine because the entire housing must be replaced upon failure of a portion thereof.
There exists a need, therefore, for a journal bearing assembly which achieves improved bearing alignment, and which reduces the problems associated with using two journal bearing assemblies, and which also provides for more efficient and less expensive overhaul and repair upon the failure of bearings.